Problem: Which of these numbers is prime? ${19,\ 56,\ 63,\ 87,\ 95}$
A prime number has exactly two factors: 1 and itself. The factors of 19 are 1 and 19. The factors of 56 are 1, 2, 4, 7, 8, 14, 28, and 56. The factors of 63 are 1, 3, 7, 9, 21, and 63. The factors of 87 are 1, 3, 29, and 87. The factors of 95 are 1, 5, 19, and 95. Thus, 19 is a prime number.